Soil moisture refers to the amount of water held in the pores of soils. It acts as an interface between atmospheric processes of precipitation and evaporation, biological processes such as photosynthesis and decomposition, energy balances of near surface land temperatures, and hydrological processes that determine water supplies (Figure 1).
Soil moisture varies across landscapes and across soil depths, based on conditions such as soil type (clay vs. loam vs. sand), how tightly packed the ground is, and more. Measurements of soil moisture are typically taken either near the surface (less than 10 cm depth) or in the rooting zone (10 cm to 200 cm depth).
There are many ways of measuring soil moisture, including: in-situ data collection (instruments placed in the ground), remotely sensed data collected by airplanes, satellite-based measurements, and modeled measurements–where data such as temperature, rainfall, and vapor pressure deficit are used to estimate soil moisture. Which approach is best for understanding soil conditions depends on the geographic scale under consideration and how the data are being applied. For example, in-situ measurements may be accurate for their location but may not give an accurate picture of conditions for the broader region because soil moisture is so variable spatially. Conversely, satellite data offer a general idea of soil moisture values regionally but may not be accurate at the scale of an agricultural field or single hillslope. For many purposes, a mix of observation types and modeling is used.
Understanding this under-measured variable is relevant for agricultural planning, fire risk assessment, water supply forecasting, drought monitoring, and flood and landslide early warning systems. Consequently there is growing interest in amassing accurate and widespread data records on soil moisture for the Colorado River Basin.
Monitoring soil moisture conditions is critical for water management and planning, in particular. Soil moisture is closely linked to drought. In hot, dry conditions water evaporates directly from the soil, and water loss from plants during respiration (transpiration) increases. This loss of water from the soils (evapotranspiration) not only impacts plant survival during the growing season, it also plays a role in runoff to streams and rivers. Soils act much like a sponge – a dry soil will take up a larger amount of water than a wet soil, which leaves less rain or melting snow to reach streams and rivers as runoff.
In regions with snowpack-driven hydrology, the amount of water remaining in the soil as the ground freezes in winter is tied to the amount of water soils take up during snowmelt in the spring. In this way, soil moisture creates a link between the prior year’s precipitation and the coming year’s water supply. As temperatures in the basin continue to warm, increased evapotranspiration may lead to decreases in soil moisture, which translates into decreased runoff and reduced water supplies.
Data and tools
A subset of the NRCS SNOTEL and SCAN stations are equipped with soil moisture sensors. The NRCS Interactive Map allows users to view current conditions at each station by selecting the desired measurement (e.g., soil moisture value at 20 in. depths). Navigate to individual stations to access data on previous soil moisture conditions.
The Colorado River Basin Forecast Center provides modeled soil moisture data (as opposed to single point observations) in map formats. Users can navigate to see either current conditions as compared to a percent normal value or may look at static, gridded maps of modeled soil moisture data from previous months (as in Figure 2).
The NASA GRACE-FO website provides maps of gridded soil moisture conditions for the contiguous United States based on wetness percentile. Maps are updated weekly.
The National Coordinated Soil Moisture Monitoring Network (NCSMMN) is a recently initiated, multi-institutional program that seeks to integrate multiple forms of soil moisture data from across the United States. The program is application focused, seeking to employ targeted coordination to guide network establishment, data integration, and develop user-focused data products. Links are provided to soil moisture data and maps derived from in-situ, satellite, or modeled data.
State of the Science Report
Chapter 5.4 of the State of the Science report, describes soil moisture and monitoring data in greater detail.